GEOLOGICAL SURVEY OF THE TEEEITOEIES. 169 



tained carbonate of lime, and alnmina probablj" as a sulpbate. There 

 was no perceptible evolution of gas. In the course of the stream there 

 ■was a deposit of lime, small in quantity, incrusting grass, moss, and 

 twigs. 



About a mile east of the fort I found a number of hills, whose bases are 

 fine-grained red sandstones of very free quality. It would make a very 

 good ornamental building-stone. The rocks that succeed and lie upon 

 them are Jurassic limestones, containing an abundance of fossils. We 

 left Fort Hall on the 23d of June, and until the 28th were in the Snake 

 Eiver Valley, a wide plain covered with sand and sage-brush. For 

 ninety miles nothing else was passed over save here and there exposures 

 of dark basaltic rock, which seems to be spread out over the entire plain. 

 At some time, during or since the Tertiary period, the plain must have 

 been flooded with molten lava, which came, in all likelihood, from sev- 

 eral points of eruption. As we came down the Port Neuf Eiver we 

 could see in the distance what appeared to be an old crater, and on our 

 way across the Snake River Basin we passed another. 



At Eagle Eock we crossed Snake Eiver on Taylor's bridge. The 

 river here has cut a narrow gorge through the rock, forming quite a 

 canon. The rock rises 10 feet above the level of the water. The cur- 

 rent is very swift. The rock shows the hexagonal columns, so charac- 

 teristic of the cooling of the molten mass. At " Hole-in-the-Eock," on 

 Dry Creek, we had an opportunity of proving that the lava extends 

 over the valley like a crust, for the most part at least simply, and not 

 in the form of dikes. Here we visited a cave, which has been formed 

 by the water flowing beneath the basalt and washing out the sand. 

 The entrance to the cave is formed by a falling in of the crust. Clam- 

 bering down over the broken fragments, we discovered seven chambers. 

 There were two entrances, one to the northwest and the other to the 

 southeast. In the flrst-named direction we found three chambers, each 

 about 25 feet in height and 200 feet in diameter, they being almost cir- 

 cular. The chambers are separated from each other by loose, fallen 

 rock. After penetrating as far as possible we retraced our steps, and 

 were about leaving the place, when we discovered an aperture just 

 large enough to admit oiie at a time, leading toward the southeast. 

 Entering this we found four chambers separated from each other by 

 piles of loose, fallen rock, as in those on the opposite side. Instead of 

 being circular these were oblong in shape, each being about 300 feet in 

 length and 150 feet wide, the height being 20 feet. Each succeeding 

 chamber is somewhat lower than the preceding. The roof is arched 

 and composed of dark basaltic rock. From it there hang innumerable 

 small stalactitic formations, caused by the percolation of the water 

 through the rock. There are also numbers of air-bubbles in the rock, 

 which hang from the roof in drop-like processes, forming points for the 

 formation of stalactites. The bottom of the cave is sandy ; and in a 

 hole dug to the depth of 20 feet, it was observed to be distinctly strat- 

 ified, showing it to have been deposited by water. That this condi- 

 tion extends over the whole valley is further presumable, from the fact 

 that a considerable number of the streams flowing through it sink and 

 are lost to sight. Their disappearance is easily accounted for by their 

 flowing underneath this crust. 



On the 28th of June we left the Snake Eiver Basin, and entering 

 Beaver Head Canon, began to ascend on our way across the main divide 

 of the Eocky Mountains. The igneous rocks were still present. At the 

 mouth of the caiion we passed an isolated hill, composed of schistose, 

 or slaty phonolite, each layer being one-eighth of an inch in thickness 



